1. Field of the Invention
The present invention relates to a level sensor, a method for determining a height map of a substrate, and a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The surface of a substrate on which a pattern should be projected is usually not completely flat. Moreover substrates can show thickness variations of several microns. This flatness and thickness variations of the substrate surface may result in incorrect projection of the pattern, for instance due to focus errors or imaging errors.
To correct for unflatness and thickness variations of a wafer, it has been proposed to provide a level sensor, preferably integrated in the lithographic apparatus. Such level sensor may be used to determine a height map of a substrate, before a pattern is projected on the substrate. This height map may subsequently be used to correct the position of the substrate during projection of the pattern on the substrate.
In a known embodiment a multiple spot level sensor using optical triangulation is provided to determine a height map of the substrate. The known embodiment comprises a projection unit with nine light sources to project nine measurement beams on the substrate, and a detection unit comprising nine detection sensors each arranged to receive one of the measurement beams after reflection on the substrate surface. On the basis of the measurement signals provided by the detection sensors a height level of the substrate surface may be determined.
However, a substrate surface may comprise areas where the height level of the substrate cannot be determined, for example close to the edges of the substrate, or where the height level of the substrate surface cannot be determined with high accuracy, for instance at scribe lanes. When a measurement beam partly falls on one of these problem areas, it is desirable to disregard the height level obtained with the associated detection sensor. This has the result that the whole measurement area is disregarded although only a part of this measurement area is a problem area. When the presence of a problem area is not recognized or acknowledged, the calculated height level of the spot wherein the problem area resides may be incorrect leading to a less accurate height map, and as a result in overlay errors and/or focus errors due to incorrect or suboptimal leveling during the lithographic process.
To increase the area to be measured, U.S. Pat. No. 7,704,826, which is incorporated by reference herein, discloses a level sensor comprising a projection unit comprising nine light sources to project nine measurement beams on the substrate surface and nine detection units arranged to receive the nine measurement beams after reflection on the substrate surface. The detection unit comprises nine two-dimensional arrays each comprising a plurality of cells. Each of the nine measurement beams is received by one of the cell arrays. A selection unit is provided to select a number of the cells of the cell array as available cells. The available cells are used to obtain a measurement signal representative for a height level of the substrate surface. The available cells are selected to exclude problem areas on the substrate surface.